In recent years, refrigerators have been broadly utilized not only for various kinds of freezers and air conditioners but also for cooling systems and temperature control devices of industrial instruments and machines. Various refrigerators having a suitable size or a suitable structure have been developed. Generally, fluorocarbon refrigerants are used as a refrigerant in compression type refrigerators such as reciprocating, turbo and rotary type ones.
Lubricating oils used for these compression type refrigerators must have excellent thermal resistance as well as characteristics such as lubricating property, miscibility with fluorocarbon refrigerants, etc. Particularly, in recent years, it has become more and more necessary to use refrigerator oils having good stability, since the operating temperature of refrigerators rises for the reason that the design of refrigerators has been directed toward small-sized ones. Further, characteristics required for refrigerator oils are diversified. For example, low viscosity oils are required for reducing the fluid friction, while high viscosity oils are required for improving lubrication and sealing of the high temperature parts of small-sized refrigerators having high performance and rotary type refrigerators. Further, it is necessary to use refrigerator oils having non-toxic and non-hazardous properties in order to protect workers in factories and refrigerator repair workshops, because the refrigerator oil sometimes comes in contact with the workers when filling refrigerators with the refrigerator oil.
It has been known that fluorocarbon refrigerants such as R-11 (JIS K1520, trichloromonofluoromethane), R-12 (JIS K1517, dichlorodifluoromethane), R-22 (JIS K1519, monochlorodifluoromethane) and R-113 (JIS K1528, trichlorotrifluoroethane), etc., generally have high individual thermal stability. However, the degree of stability varies among them. For example, R-14 (tetrafluoromethane) is the stablest and does not undergo any decomposition even if heated to 400.degree. C. for 500 hours. R-11 is the least stable and does not decompose at less than 200.degree. C. and only 2.5% thereof decomposes at 200.degree. C. after 1 year. Refrigerator oils generally have high thermal stability in their pure form, in air, or in an inert atmosphere. Although the degree of stability varies among different refrigerator oils, the decomposition temperature is generally more than 300.degree. C. However, if the fluorocarbon refrigerant and the oil are present together, they are chemically activated. Further, when they are subjected to a catalytic function of metal and influenced by contaminants such as air or water, etc., they undergo decomposition or reactions at lower temperatures. Consequently they cause corrosion of metal and formation of sludges. Furthermore, the formed carbonaceous deposits or sludges sometimes cause troubles such as lowering of compression efficiency and seizure (ASHRE JOURNAL, August 1962, page 59 and ASHRE JOURNAL, November 1967, page 54).
Mineral oil type refrigerator oils contain active compounds in a very small amounts. These active compounds include sulfur compounds, nitrogen compounds and oxygen compounds which readily react with fluorocarbon refrigerant. These active compounds are removed by refining to improve thermal resistance. Generally known refining processes include solvent extraction, hydrogenation, treatment with sulfuric acid, treatment with clay and dehydrating treatment. A combination of these processes are generally carried out. Synthetic oils are generally treated with clay or dehydrating treatment in order to remove the impurities.
In order to satisfy various characteristics required for the refrigerator oil, various kinds of base oils must be utilized. However, some kinds of oil cannot be economically refined. Accordingly, additives may be effectively utilized in some instances.